Fix UnboundLocalError for bytes payload and quoted-printable cte.

Author: skv0zsneg

Lib/email/message.py

@@ -313,6 +313,8 @@ def get_payload(self, i=None, decode=False):
                 # If it does happen, turn the string into bytes in a way
                 # guaranteed not to fail.
                 bpayload = payload.encode('raw-unicode-escape')
+        elif isinstance(payload, bytes):
+            bpayload = payload
         if cte == 'quoted-printable':
             return quopri.decodestring(bpayload)
         elif cte == 'base64':